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Geology is the Way

Olivine

Orthorombic

(Mg, Fe)2[SiO4]

Olivine is a ferromagnesian nesosilicate that occurs in mafic and ultramafic rocks in the Earth’s crust. It is the most abundant mineral in the Earth’s mantle, constituting more than 50% of the upper mantle. The name ‘olivine’ indicates its distinctive olive/bottle green to yellowish color and was coined in 1789 by Abraham Gottlob Werner, substituting many other names that were used at the time, including smaragdus and beryllos (Pliny, 79) and chrysolit (Johan Gottschalk Wallerius, 1747) [Mindat].

Structure and chemistry
The members of the olivine group are nesosilicate minerals, whose crystal structure is characterized by isolated SiO4 tetrahedrons surrounded by octahedral M-sites that contain (Mg,Fe)2+ cations, each coordinated by six oxygens. The SiO4 tetrahedrons alternate in the structure, pointing alternatively up and down perpendicularly to the c-axis and along the a-axis. M-sites are distinguished into a regular set of octahedral sites (M1) and a series of slightly larger, distorted octahedral sites (M2). M-sites form long chains of octahedrons elongated parallel to c. Each oxygen in the structure is bound to one Si atom and three M-site cations. 

olivine crystal structure
Scheme highlighting the crystal structure of olivine on the (100) plane (left) and (001) plane (right). Modified after Deer, Howie & Zussmann.

Strictly speaking, olivines represent a solid-solution between forsterite (Fo: Mg2SiO4; named in honor of Adolarius Jacob Forster) and fayalite (Fa: Fe2SiO4; named after Fayal Island, Azores). Forsterite and fayalite form a continuous solid solution, driven by the substitution of Mg for Fe in the octahedral sites. Fe2+ has a small preference for the M1 site. This solid solution is commonly denoted using the % of end members in the solution. For instance, an olivine with 30% forsterite and 70% fayalite is indicated as Fo30, Fa70, or Fo30Fa70. Similarly to the plagioclase series, different names for compositional intervals of olivine have been proposed (chrysolitehyalosideritehortonolite, and ferrohortonolite; see figure). However, these names are not commonly used.
Natural olivines show limited substitution of (Fe,Mg) for Ca, because of the larger ionic radius of Ca that does not allow it to enter the structure of forsterite – fayalite easily. Ca-bearing olivines, monticellite (CaMgSiO4), kirschsteinite (FeMgSiO4), and larnite (Ca2SiO4), have larger unit cells compared to ferromagnesian olivines, with larger octahedral sites that allow them to accommodate the Ca cation in their structure. Ca-bearing olivines are very rare and, in nature, occur mostly as accessories in Ca-rich silica-undersaturated rocks (e.g. carbonatites) and metasomatic rocks and skarns deriving from impure limestones. Other cations that may be present within forsterite-fayalite are Mn, Cr, Ni, and Fe3+. Cr may occur as thin lamellae of exsolved chromite within olivine.


The gem variety of olivine: peridot. Note the squat prismatic habit. Size: 3.4 x 2.5 x 1.9 cm. Locality: Naran-Kagan Valley. Kohistan, Pakistan. Photo by Robert M. Lavinsky.

Properties
Habit: stocky prismatic-tabular, ‘barrel-shaped’ grains
Hardness: 6.5 – 7
Density: 3.2 (forsterite) – 4.4 (fayalite) g/cm3
Cleavage: {010} poor, {100} very poor
Twinning: {011}, {012}, {031}
Color: olive green, green, yellow-green, yellow-amber
Luster: vitreous
Streak: colorless
Alteration: serpentine, iddingsite, bowlingite, chlorophaeite
In thin section…
α(//b): 1.635 (forsterite) – 1.827 (fayalite)
β(//c): 1.651 (forsterite) – 1.869 (fayalite)
γ(//a): 1.670 (forsterite) – 1.879 (fayalite)
2Vγ: 82° (forsterite) – 134° (fayalite)
Color: colorless (forsterite) to pale yellow (fayalite)
Pleochroism: none (forsterite); α = γ pale yellow to β orange yellow, reddish brown (fayalite)
Birefringence (δ): 0.035-0.052 (high interference colors, higher in fayalite)
Relief: high
Optic sign: + or –
[Mindat]

olivine classification diagram
Ternary diagram showing the chemical variability of the olivine group members. Two main solid solutions occur: forsterite-fayalite and monticellite-kirschsteinite. Common rock-forming olivines belong to the forsterite-fayalite series. Ca-olivines are rare in nature.

Field features

olivine crystal habit
Sketch illustrating the crystal habit and position of crystal axes in forsterite and fayalite olivine. Modified after Deer, Howie & Zussmann: An Introduction to the Rock-Forming-Minerals.

Fresh olivine in the field can be identified based on (1) its characteristic stocky habit with six-sided or eight-sided ‘barrel-like’ sections, (2) distinctive bottle green-yellowish transparent color, and (3) vitreous luster, similar to quartz. However, olivine is very susceptible to alteration and oxides easily, getting replaced by mixtures of minerals with opaque black, green or reddish colors. Altered olivine is still relatively easy to distinguish from other mafic minerals like pyroxene and amphibole, because it lacks any well-visible cleavage and metallic luster.

olivine basalt
Basalt with slightly weathered olivine phenocrystals showing the typical green color and stocky habit. Oahu, Hawaii, USA. Width of sample 6 cm. Photo by Siim Sepp.
olivine basanite
Weathered olivine crystals with grey to brown colors, preserving the distinctive habit, associated with black, prismatic pyroxene crystals. Basanite from La Palma, Canary Islands, Spain. Photo by Siim Sepp.
peridotite xenolith
Transparent olivine crystals (green) in a peridotite nodule in basalt from the San Carlos Indian Reservation, Arizona, USA. Photo by Aomai.
olivine marble
Olivine from a metamorphosed impure carbonate rock. Olivine (here showing very dark green to black color) show well-developed crystal faces and barrel-like shape. Width of sample is 9 cm. Photo by Siim Sepp.
olivine sand
Olivine sand is extremely rare, because of how easily olivine alters. However, strands of green sand exist on volcanic islands, where olivine grains eroded from basaltic rocks accumulate on the shore. Papakolea, Hawaii, USA. Width of view 20 mm. Photo by Siim Sepp.

Olivine in thin section
Magnesian forsterite, the most common type of olivine in nature, shows high relief and appears colorless at PPL. The typical habit of olivine is quite stocky, with barrel-like six-sided or eight-sided sections being very common. However, in some volcanic rocks olivine may form prismatic or even acicular grains (skeletal grains like spinifex olivine). Olivine may show a set of imperfect cleavage planes oriented parallel to (010) [parallel to the long axis of the ‘barrels’] and, less commonly, a second set of very poor (100) cleavage planes, the latter largely occurring as imperfect fractures. 
At CPL, olivine shows high third order interference colors. Along the forsterite-fayalite series, refraction indices, birefringence, and relief increase with increasing content of Fe. Fayalite-rich olivine appears pale yellow and pleochroic (yellow to orange).
Alteration of olivine is common. Olivine may alter to serpentine or aggregates of minerals that appear reddish (iddingsite, chlorophaeite) or greenish (bowlingite) in thin section [More details below in this page].

CPL
CPL
CPL
PPL
PPL

Round olivine (high relief at PPL, high interference colors at CPL) and chromite crystals (opaque) surrounded by a calcite groundmass (high grey interference colors at CPL). Kimberlite from Bloemfontein, South Africa. Width: 7mm. Photo © Alessandro Da Mommio (alexstrekeisen.it).

olivine phenocryst
Euhedral olivine crystal in a basalt, showing the typical stocky habit and high interference colors. It is also possible to observe (010) cleavage planes (lower-left to upper-right, parallel to the long axis) and the (100) fracture (upper left to lower right). The surrounding groundmass contains abundant plagioclase crystals. Etna Volcano, Italy. CPL. Width: 7mm. Photo © Alessandro Da Mommio (alexstrekeisen.it).
olivine plagioclase basalt
Olivine (high interference colors) and plagioclase (grey interference colors, polysynthetic twinning). Basalt from Etna Volcano, Italy. CPL. Width: 7mm. Photo © Alessandro Da Mommio (alexstrekeisen.it).
olivine basalt
Euhedral olivine crystals surrounded by a plagioclase-bearing groundmass in a basalt from Iceland. CPL. Width: 2mm. Photo © Alessandro Da Mommio.
olivine marble
Round olivine crystals (some altered to serpentine) and a large muscovite crystal (lamellar) surrounded by calcite in an olivine marble. CPL. Width: 7 mm. Photo © Alessandro Da Mommio.

Alteration of olivine
Olivine is very susceptible to hydrothermal alteration and weathering and is easily replaced by phyllosilicates, oxides, and hydroxides. During ocean floor metamorphism, olivine commonly alters to serpentine and brucite, as well as talc and various carbonates. Some distinctive alteration products of olivine are named iddingsite, bowlingite, and chlorophaeite. These are not minerals, but aggregates of alteration products that commonly produce pseudomorphs over olivine.

Iddingsite: reddish brown replacement of olivine consisting of smectite, chlorite, goethite, and hematite.
Bowlingite: greenish alteration of olivine consisting of smectite, chlorite, serpentine, talc, white mica, and quartz.
Chlorophaeite: similar to iddingsite but more variable in color and containing less Fe3+.

CPL
CPL
CPL
PPL
PPL

Mesh texture: relics of olivine (high relief at PPL, high interference colors at CPL) in optical continuity surrounded by fibrous serpentine (transparent at PPL, grey at CPL). Width: 2mm. Photo © Alessandro Da Mommio.

CPL
CPL
CPL
PPL
PPL

Iddingsite alteration (orange/reddish) on olivine rim. Width: 2 mm. Photo © Alessandro Da Mommio.

CPL
CPL
CPL
PPL
PPL

Olivine crystal completely substituted by bowlingite (pseudomorphed) in a basalt from India. Bowlingite appears greenish at PPL and shows high interference colors at CPL. Width: 2 mm. Photo © Alessandro Da Mommio.

Occurrence
Olivine is the main constituent of peridotites, in which it occurs with a forsteritic composition. Dunites are a subgroup of peridotites that consist almost entirely (> 90%) of olivine. Olivine is present also in a wide range of mafic and ultramafic igneous rocks, like cumulates, kimberlites, gabbros, basalts, and komatiites. Mg-rich olivine is incompatible with quartz and, in presence of enough silica, magmas produce pyroxenes. By contrast, Fe-rich olivine (fayalite) is stable in presence of quartz and may occur in Fe-rich alkaline granites, syenites and their volcanic counterparts.
Olivine is commonly replaced by serpentine during ocean floor metamorphism of peridotites, which react with hydrothermal ocean-derived water in Mid-Ocean ridges, forming serpentinites. Serpentinites can react again to form olivine if they reach sufficient heat and pressure during later metamorphism: this may happen in subduction zones and orogenic settings. Olivine can also form during high temperature metamorphism of impure carbonates and carbonate-bearing ultramafic rocks, from reactions between quartz and amphibole or involving the decarbonation of magnesite or dolomite. Fayalite may form in a similar fashion, from the reaction between quartz and Fe-rich carbonates (siderite, ankerite).

References and Further Reading
Buening, D. K., & Buseck, P. R. (1973). Fe‐Mg lattice diffusion in olivine. Journal of Geophysical Research78(29), 6852-6862.
Donaldson, C. H. (1976). An experimental investigation of olivine morphology. Contributions to mineralogy and Petrology57(2), 187-213.
Goetze, C. (1978). The mechanisms of creep in olivine. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences288(1350), 99-119.
Raleigh, C. B. (1968). Mechanisms of plastic deformation of olivine. Journal of Geophysical Research73(16), 5391-5406.
Roeder, P. L., & Emslie, R. (1970). Olivine-liquid equilibrium. Contributions to mineralogy and petrology29(4), 275-289.
Siever, R., & Woodford, N. (1979). Dissolution kinetics and the weathering of mafic minerals. Geochimica et Cosmochimica Acta43(5), 717-724.
Simkin, T., & Smith, J. V. (1970). Minor-element distribution in olivine. The Journal of Geology78(3), 304-325.

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